When drilling a borehole, such as a hydrocarbon well, it is necessary to obtain information of the formations being drilled. While some information can be derived from the drilled materials returned to the surface, it is often necessary that measurements be made in-situ or on larger samples in order to obtain the necessary information. Certain properties can be measured by lowering a tool into the well and making non-intrusive measurements while the tool is moved vertically. This technique is known as electrical logging. The measurements made by the tool are returned to the surface as signals either in a wire cable or in mud pulses while drilling where they can be detected and analyzed. Consequently, the techniques are also known as wireline logging or logging/measurement-while-drilling.
Commonly measured properties relate to inherent properties of the formation such as electro-magnetic, nuclear and sonic behavior of the formation. Such measured properties allow the determination of formation resistivity, natural gamma-ray emission and sonic wave velocity. However, wireline logging and logging/measurement-while-drilling may not be able to identify weak subsurface intervals that are inter-bedded within competent formations, since the logging is typically conducted overbalance, which may mask the logging measurements of weak intervals.
In situations where identifying such weak intervals is critical, for example, to avoid running sand control by avoiding perforating the weak intervals, the approach which has been used previously includes coring the entire reservoir to retrieve rock cores. The rock cores are returned to the surface for identification of the intervals and for laboratory testing. This approach, however, is expensive and time-consuming. Further, it does not allow a continuous logging approach in which measurements are made continuously as the tool is moved along the borehole.
There is therefore a need to provide a tool which can identify weak subsurface intervals traversed by a borehole on a continuous basis and in a continuous logging operation.